**Core Concept**
The question tests the understanding of a common cause of "bloody" vomitus in neonates, which is related to the **gastrointestinal tract** and **maternal factors**. This condition is often due to the ingestion of maternal blood during delivery.

**Why the Correct Answer is Right**
The correct answer is related to the swallowing of maternal blood during birth, which can lead to **hematemesis** or vomiting of blood. This blood can come from the mother's genital tract during delivery.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect as it is not directly related to the common cause of "bloody" vomitus in neonates.
**Option B:** Incorrect as it does not specify the correct cause.
**Option C:** Incorrect as it is not a common cause of "bloody" vomitus.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the vomiting may appear more like coffee grounds than fresh blood due to digestion.

**Correct Answer:** D. Swallowed maternal blood.